1. Field of the Invention
The present invention relates to an implantable cardiac stimulating device.
More precisely, the invention concerns a dual chamber pacemaker, i.e. a pacemaker which is adapted to stimulate and/or sense both an atrium and a ventricle of the heart. In particular, the invention concerns a, pacemaker which includes means to deliver stimulating pulses to the atrium and to the ventricle and means for sensing events in the atrium and in the ventricle. The means arranged for sensing events in the ventricle includes means for sensing an evoked response of the ventricle to a delivered stimulating pulse. Furthermore, the cardiac stimulating device includes means which may be arranged to deliver a back-up pulse to the ventricle if the means arranged to sense an evoked response does not sense any evoked response to a delivered stimulating pulse. Moreover, the cardiac stimulating device includes a control system which is arranged to operate the device in at least a first and a second manner, wherein according to the first manner no stimulating pulse is delivered to the atrium and according to the second manner, stimulating pulses are delivered to the atrium. Such a control system may work in response to the means for sensing events in the atrium, such that a pulse is delivered to the atrium if and only if no event is sensed in the atrium within the atrial escape interval.
2. Description of the Prior Art
For a general overview of the function of an implantable cardiac stimulating device, see, for example, U.S. Pat. No. 5,873,895, columns 2–5.
U.S. Pat. No. 5,476,487 describes the function of an implantable cardiac stimulating device which emits a back-up pulse to the ventricle when no evoked response is detected to an applied ventricular pulse; see in particular columns 1–2.
A cardiac stimulating device which operates with such back-up pulses is hereinafter also called an autocapture pacemaker (Autocapture™ pacing system). Such a pacemaker may operate in the following manner. Shortly after a ventricular pulse has been delivered, for example 5–20 ms after the delivery, the pacemaker begins sensing whether a response occurs in the ventricle. This sensing may, for example, take place during 40–60 ms. If a response is sensed during this time interval, then no back-up pulse is emitted. However, if no evoked response is sensed, then a back-up pulse is emitted. The back-up pulse is preferably emitted immediately at the elapse of this time interval. The back-up pulse is usually emitted with increased output power in order to secure capture of the heart. For example, the normal ventricular pulse may be emitted with an amplitude of 1.5 V and the back-up pulses may be emitted with an amplitude of 4.5V.
In order to facilitate the understanding of the present invention, some basic working principles of a normal dual chamber pacemaker according to the prior art will now be described with reference to FIG. 1. FIG. 1 schematically depicts a timing/waveform diagram indicating cardiac and pacemaker events. A represents a stimulating pulse delivered to the atrium. V represents a stimulating pulse delivered to the ventricle. P indicates the atrial depolarization, in this case a paced depolarization. R indicates ventricular depolarization. R is thus in this case the evoked response to a delivered ventricular pulse V. T represents the ventricular repolarization. A heart cycle HC, as this concept is used in the present description and in the following claims, starts with a paced or sensed atrial event and ends at the subsequent paced or sensed atrial event.
AVI is the so-called AV-interval. This is a pre-set, normally programmable, time interval beginning with a sensed or paced atrial event. If no ventricular event is sensed during the AV-interval, then a ventricular pulse is delivered at the end of this interval. A normal AV-interval may have a length of, for example, 150–210 ms. AEI indicates the atrial escape interval. This is a predetermined, normally programmable, time interval which starts with an atrial or a ventricular sensed or paced event. If no atrial event has been sensed at the end of this pre-set atrial escape interval, then a stimulating pulse is delivered to the atrium at the end of this interval. BP is the ventricular blanking period. This is a short time interval after the delivery of an atrial pulse during which interval the ventricular sense amplifier is disabled and therefore cannot detect any signals. If there were no ventricular blanking period, then the ventricular sense amplifier could sense a paced event in the atrium and interpret this as a ventricular event. The pacemaker normally also operates with a similar atrial blanking period (which is not shown in FIG. 1).
Additionally, the following concepts, which are not shown in FIG. 1, will be explained. The “ventricular biological refractory period” is a period following a ventricular event (an R-wave) during which the heart will not respond to a ventricular stimulation. The “vulnerable period” is a part of the heart cycle, normally coincident with a part of the T-wave, during which a stimulation pulse may possibly cause repetitive rhythms such as, for example, tachycardia or ventricular fibrillation. The “ventricular complex” includes the ventricular depolarization and a short time before and after this depolarization.
The above explanations of the aforementioned concepts are not intended as absolute definitions, but rather as examples of how these concepts are normally used. Furthermore, it should be noted that in FIG. 1 no back-up pulses are shown. The function of an autocapture pacemaker will thus involve additional time intervals, as described above in connection with the explanation of the function of a pacemaker working with back-up pulses.